Thoriated cathodes and method for making the same

ABSTRACT

THORIATED HOT CATHODES IN WHICH THE METAL CARBIDE SUCH AS W2C OR M02C IS TRANSFORMED FROM HEXAGOANL TO BODY CENTERED CUBIC MODIFICATION. THIS IS DONE AT AN ELEVATED TEMPERATURE BY WAY OF A SOLID-STATE REACTION, IN THE PRESENCE OF A SUITABLE METAL CATALYST.   D R A W I N G

Aug.

.75 amp I KONCZ THORIATED CATHODES AND METHOD FOR MAKING THE SAME Filed April 23, 1958 0, .'75 amp UnitedStates Patent O 3,600,334 THORIATED CATHODES AND METHOD FOR MAKING THE SAME Istvan Koncz, Budapest, Hungary, assignor to Siemens Aktiengesellschaft, Berlin and Munich, Germany Continuation-impart of application Ser. No. 384,500, July 22, 1964. This application Apr. 23, 1968, Ser.

Int. Cl. H01b 1/-00 U.S. Cl. 252-515 10 Claims ABSTRACT F THE DISCLOSURE Thoriated hot cathodes in which the metal carbide such as W2C or MoZC is transformed from hexagonal to body centered cubic modification. This is done at an elevated temperature by way of a solid-state reaction, in the presence of a suitable metal catalyst.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of co-pending application Ser. No. 384,500 filed July 22, 1964.

BACKGROUND OF THE INVENTION The present invention relates to so-called hot cathodes for use in electric discharge vessels, particularly cathodes in which the active component is thorium, and to methods for making the same.

Cathodes containing thorium as the active component are well known in the art and are presently used in various pieces of electronic equipment, such as electron tubes, as well as in gas-filled discharge vessels.

Normally, thoriated cathodes fall into four types, as follows:

(1) So-called film cathodes, which are cathodes made of tungsten wire containing thorum oxide, the active emitter surface being formed by subjecting the incandescent filament to a high-temperature treatment.

(2) Thoriated tungsten cathodes having a carburized surface, which are made from tungsten wire containing thorium oxide. The wire is heated with carbon, or gaseous compounds of carbon, so as to form a carbide layer, the carbide layer being about to 30% of the cross section and consisting of hexagonal tungsten carbide whose chemical formula is W2C. The function which this carbide layer plays will be referred to below in detail.

(3) Massy cathodes, mainly pasty or reserve-type cathodes, whose active mass consists of thorium oxide, which may, if desired, be admixed with a metal that melts at a high temperature and with a reducing agent such as, for example, carbon.

(4) So-called cermet (ceramic-metal) cathodes, that is to say, sintered cathodes, made up of a metal that is diicult to melt and thorium oxide, with or (without reducing agents, according to powder-metallurgical techniques.

The film-type cathodes described in 1) are very sensitive in that they are easily poisoned by ion bombardment or by impurity gases, so that, for practical purposes, such cathodes are no longer in use.

The thoriated tungsten cathodes described in (2) have a long life expectancy but are very brittle and are quite sensitive to impurity gases. In general these cathodes have the same operating values insofar as maximum current density and power output are concerned as in the case of the film-type cathode described in (1). Therefore, the only signilicance that was assigned to the carbide layer was that it had better reducing properties with respect to thorium oxide, thereby to facilitate the diffusion of the thorium.

The massy cathodes described in (3) were developed based on the supposition that metallic thorium had to be present in order to obtain good emission, i.e., that part of the thorium oxide had to be reduced to the metallic thorium. It was assumed that heavy metals such as, for example, tungsten or molybdenum, as well as other difficultto-melt metals, could reduce the thorium oxide to the metal. As is now known in the art, this is not so, or if such reduction does occur, it twill, at the usual cathode temperatures, do so to a negligible extent.

The cermet cathodes described in (4), i.e., the sintered cathodes made in accordance with powdered metal techniques, are also based on thorium oxide and one or more diicult-to-melt metals. In general, these cathodes are made without carburizing, but in case there is carburizing, all that is obtained s the known hexagonal metal carbide phase.

SUMMARY OF THE INVENTION It is the primary object of the present invention to provide a hot cathode which overcomes the drawbacks of heretofore known hot cathodes, namely, to provide a hot cathode which is capable of putting out high power, which has a high emission current density and a long life expectancy, which is insensitive to poisoning and which, moreover, has good mechanical strength and which is stable.

It is another object of the present invention to provide a new modification of metal carbides which is used as the constituent in a hot cathode, thereby to produce better emission, higher stability and better mechanical properties.

It is still a further object of the present invention to increase the maximum permissible current density which, in the case of conventional thoriated cathodes is, at the usual operating temperature, about 2-3 A./cm.2- to about double that which was heretofore possible, namely, to about 5-6 A./cm.2 at the same operating temperature, while simultaneously reducing the costs of the cathode.

In accordance with the present invention, the above objects are achieved by providing a cathode of the above type in which the conventional hexagonal carbide is transformed into a new body centered cubic (b.c.c.) modification.

More particularly, the present invention resides in a cathode in which the b.c.c. carbide is produced, in an appropriate process step, by catalytically reacting a metal. This new modification is the determining component of the nished cathode. The mentioned carbide results from a carbide of a dillicult-to-melt metal, such as W or Mo, which carbide is produced by reaction with carbon or a hydrocarbon at high temperature. This carbide will have the conventional hexagonal crystal structure and is then transformed by way of a catalytic solid-state reaction which converts the hexagonal carbide in the new b.c.c. modification. This reaction is carried out at high temperature in the presence of a metal such as Pt, Pd, Ru, Os, Ir, Rh, Re, Fe, Ni, Co, Cr, Ag, Mn or Cu. This heat treatment is continued until the original hexagonal carbide phase disappears and the new phase is formed. The solidstate reaction takes place at a temperature which is between about 2/3 of the melting point of the metal and the melting point of the metal, and continues until the catalytically acting metal has, for the most part, been vaporized. Remaining traces become part of the crystal lattice and act as a dopant.

It will thus be seen that, in accordance with the present invention, the carbide, which has the general formula MZC where M is a metal melting at a high temperature, preferably W or Mo, initially has a hexagonal structure and is converted, catalytically and at high temperature, into the new b.c.c. phase. This new carbide phase can be produced at the surface of a wire, strip or the like. The wire itself may be made in such a way as to contain the necessary amount of thorium, or it can be made of the particular carbide and be added to a mixture containing thorium This transformation can just as Well be carried out in the case of Mo or W wires which are alloyed with one or more difficult-to-melt metals.

The exact way in which the new carbon phase according to the present invention brings about the higher emission is not yet fully understood, but it is assumed that a semiconductor mechanism is involved, and that the b.c.c. carbide layer, acting as a semiconductive surface, brings about the conditions conducive to the increased emission. Actually, even the effect of the heretofore used hexagonal carbide phase in thoriated cathodes is not completely understood, and in order to investigate the question of whether a typically increased emission of thoriated cathodes is possible Without carbon, the following experiment was carried out:

In order to explain the part played by the carbide component in the emission, the original Langmuir measurements were repeated, with the object of withholding carbon and its compounds as far as possible from the cathode, i.e., of preventing the production of any carbide impurity on the tungsten surface. For this purpose, all the metal parts, especially the incandescent wire, were heated a number of times in a moist hydrogen atmosphere for long periods of time. The pumping unit which -was used had no lubricated connections and valves. All the glass parts were treated similarly. In addition, the carbon residues, and especially carbon monoxide, were removed by incandescent and also cold getters (titanium) of large area having high affinity for carbon. Measurement of the work function of a noncarbun'zed, commercially obtainable thoriated tungsten filament showed under the aforesaid conditions values of about 3.4-3.5 ev., which value corresponds approximately to the work function of metallic thorium. However, when very small quantities of hydrocarbons were added to the measuring arrangements by way of a breakable seal, a characteristic value of about 2.6-2.8 ev. was measured without diculty after activation.

It may be concluded from this that the production of the low work function normally attributed to the system of thorium on tungsten is dependent upon the possibility of a carbon contamination. This impurity reacts with tungsten to form tungsten carbide at the activation temperature. It is concluded that the characteristically low work function of thoriated cathodes is not a property of the tungsten-thorium system but of the system tungsten carbide-thorium.

If it is indeed a semiconductor mechanism which is responsible for the higher emission of a thoriated cathode, it should be possible to influence the emission by varying the characteristics of the semiconductor. This is, in accordance with the present invention, achieved by transforming the Structure of the carbide and influencing the semiconductor properties by the remainder of catalyst which is vaporized from the surface.

The new b.c.c. carbide phase can be produced either at the surface of a wire or the like, by converting the hexagonal carbide, or this carbide phase can be produced in a separate step from the hexagonal carbide, which can be added to a thorium-containing active mass or to a mass which is to be pressed into the shape of the cathode. This new carbide phase, hereinafter referred to as the 'q2 carbide phase, is preferably at least 5% by weight of the ThOZ, but may advantageously be increased to 50%.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the thermionic cathode produced by the process of the invention are illustrated in the drawings, in which:

FIG. 1 shows the emission curves of a conventionally carburized thoriated tungsten cathode and of a cathode produced by the process according to the present invention.

FIG. 2 is a graph illustrating the saturated emission current data of a thoriated tungsten cathode, according to the invention, during the useful life.

FIG. 3 illustrates the initial starting current of a thoriated tungsten cathode produced by the process of the invention at the exhausting state as compared with the behavior of a conventional thoriated cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. l illustrates the values of the emission current, plotted against the heating power, obtained in comparative measurements of two series of identical types of tubes manufactured under normal production conditions, the only dilerence between the two series residing in that cathodes produced by the process of the invention were used in one series and cathodes produced by the conventional process were used in the other series.

The geometrical dimensions of the cathodes are the same. Curve 1 of FIG. l shows the saturation current of the tubes comprising the cathode according to the invention, while curve 2 shows by way of comparison the emission values of the standard, normally carburized cathodes. At the rated power, the emission of the cathode exceeds the standard value by FIG. 2 shows the saturation current values, related to a cathode area of one square centimeter, of three tubes (curves 3, 4 and 5) comprising the cathode according to the invention in the course of the life test. These tubes Were operated during the tests with the maximum rated power in the 12 mHz. range, class C operation.

Despite the high specific loading-which was approximately twice that of normal cathodes-the tubes exhibit no reduction of cathode activity during the normal useful life.

FIG. 3 illustrates the behavior of the cathode according to the invention at the pump (curve 6), as compared with a conventional thoriated cathode (curve 7), at the first heating of the cathode with applied anode voltage.

Curve 6 shows that in the case of the cathode produced in accordance with the invention, a readily measurable emission current is set up at a much lower temperature than in the reference cathode, and this current remains steady as the temperature rises Without any sudden changes in activation at higher temperature, as is apparent, for example, from curve 7.

As set forth above, the cathodes according to the present invention are produced by transforming the hexagonal carbide phase into the 112 or b.c.c. modification. This is done as follows:

The cathode which contains as active metal, thorium and as a refractory metal, either W or Mo and a carbide of the refractory metal, so that the chemical composition is M2C wherein M represents the refractory metal, has the carbide in a hexagonal crystallographic structure. This structure is transformed in the presence of Pt, Pd, Ru, Os, Ir, Rh, Re, Fe, Ni, Co, Cr, Ag, Au, Mn or Cu by a solidstate reaction at elevated temperature through a catalytic action of one of these last-mentioned metals until the hexagonal carbide is completely transformed to a body centered cubic modification. The temperature applied during the solid-state reaction is above 2A of the melting point of the used catalyst metal. Consequently, the catalyst metal evaporates substantially and disappears from the surface of the carbide.

In practice, the cathode may be in the form of a thoriated tungsten filament and tungsten carbide, made by carburizing the surface of the filament by heating in a carbon-containing media.

According to another embodiment of the invention, the cathode is made by providing a separately made -filler mass which includes crushed b.c.c. carbide in fine particle form, which is admixed with thoria (thorium oxide) and to a supporting metal powder. The thus-obtained mixture is then milled so as homogeneously to distribute the same, after which the mass is applied, with a conventional binder, to the supporting cathode body.

According to still another embodiment, the cathode is a sintered cermet cathode in which a ceramic sintered body, made of ThO2, a refractory metal and a refractory metal carbide in hexagonal crystallographic structure, is made' by mixing the components and sintering them to a ceramic body, which is immersed in a solution of a salt of Pt, Pd, Ru, Os, Ir, Rh, Re, Fe, Ni, Co, Cr, Ag, Au, Mn or Cu. The thus-impregnated body is then dried and heat-treated, at a temperature above 2/3 of the melting temperature of the catalyst metal, to transform the hexagonal carbide into a b.c.c. modification.

Instead of immersing the sintered body, the same may, in accordance with another embodiment of the present invention, be exposed to a vapor of one of the catalyst metals and be heat-treated, as described above.

According to yet another embodiment of the present invention, a sintered ceramic body consisting of ThO2, of a refractory metal as supporting member, and of an already transformed metal carbide having the desired b.c.c. crystallographic structure is milled to a homogeneous powder mixture, after which the desired shape is obtained by conventional powder metallurgical methods and then sintered.

The following are illustrative and not limitative examples of the present invention.

Example I A helical cathode, made of thoriated tungsten wire containing 2% thorium oxide and having a diameter of 1.0 mm. and a rated heating voltage of 5 v., is carburized in a naphthalene vapor, with a heating current of 6.5 A., until the voltage reaches 7.95 v. The cathode is de-gassed under a high vacuum for minutes at a temperature of 2200 K., and is afterwards coatai with palladium, this being done with the use of the commercially available Palladium Plating Solution 3772 sold by Engelhard Industries. The current density during the plating was l0 ma./ cm.2 and electrical energy was supplied at a rate of 3000 ma./sec./cm.2. This resulted in a coating of 0.2 mg./cm.2. The cathode was then washed and dried.

The thus-coated cathode was then subjected to diffusion heating, under a high vacuum for one hour at a temperature of 1450 C.pyr (the subscript pyr indicating pyrometer measurements) and then for two hours at a temperature of 2000c C.Wr so as to bring about the solidstate reaction. The cathode iwas then cooled under a high vacuum and mounted in the tube of the electrical discharge vessel, which was then evacuated in the usual manner.

Microscopic examination showed the cross section of the cathode filament to be homogeneous, there no longer being a definitive line of demarcation between the core and carbide layer. Only the b.c.c. 112 structure could be found by X-ray diffraction, while traces of palladium, of the order of p.p.m., were found spectrographically,

The thus-produced tube was tested to yield a specific.

emission current of 5.2 A./cm.2.

Example II A thoriated tungsten cathode having the same characteristics as the cathode described in Example I was carburized and de-gassed, as described. After the degassing, the cathode was mounted in a glass envelope containing rhenium pentachloride. The temperaturel of the cathode was raised to 1900 C.pyr and this temperature was maintained for two hours, after which the cathode was cooled, degassed under a high vacuum and mounted in a conventional electron tube system.

The cathode was found to have the same desirable properties as those set forth in Example I. The hexagonal carbide phase was likewise found to have been eliminated.

Example III An M-shaped lamentary cathode made of thoriated tungsten wire having a diameter of 216 am. and having a rated heating current of 3.2 A. and a voltage of 10 v. was carburized in naphthalene vapor, the current applied being 4.75 A. until the heating voltage rose from 17.2 v. to 23.2 v. The cathode was then de-gassed under a high vacuum and subsequently coated cataphorically with thorium oxide from a suspension containing 5 g. ThO2/ 100 cm.3 until the thorium oxide attained a thickness of 30 am. After this coating, the tube was assembled.

The thus-produced cathode was a tungsten cathode coated with thorium oxide. This oxide coating of the carburized body, which consists of b.c.c. modification, remained white and produced a constant emission during its lifetime. This is in contradistinction to conventional thorium oxide cathodes which are applied on tungsten bodies and which turn black, either in the course of use or even while they are being manufactured.

Example IV A molybdenum wire having a diameter of 1 mm. was recrystallized under a high vacuum at 2400 0.1m. The wire then was carburized, to about 50% of its cross section, in a flowing atmosphere of xylene and hydrogen. The wire was then coated galvanically with nickel, in a conventional acidic nickel solution, until the coating reached 0.3 mg./cm.2. The thus-coated wire was then annealed under a high vacuum for one hour at 1300 C.pyr and then for two hours at 1800 Op, after which the wire was cooled under high vacuum.

The wire was then broken into pieces and ground in a vibratory grinder until the average particle size of 30 am. was reached. This powder was then milled with thorium oxide (20 g. of powder to 15 g. of thorium oxide) and this mass was admixed with a binder (plexiglass, chloroform) and used as filler mass for reserve-type cathodes of mercury vapor lamps. The cathodes were found to have good stability and a long useful life.

Example V The powder consisting of Mo and @M020 as described in Example IV was mixed with thorium oxide, two parts by weight of this metal-carbide powder to one part by weight of ThO2, as well as with 8 parts by weight of Mo powder. The mixing was done in a ball mill. The cathode body-in this case a cylindrical bodywas pressed from this powder by means of conventional metallurgical processes. The cathode body was sintered under high vacuum at 2300 C.pyr and then finished by grinding and built in as the finished cathode.

Example VI A conventionally carburized tungsten wire containing 2% ThO2 was coated with platinum to 0.1 mg./cm.2 and then annealed for one hour at 1450 C.pylr and for two hours at 0 CTW. The wire was then broken into pieces and ground and then milled with ThOz, 10 parts by weight of the Mo powder to 1 part by weight of ThOz. The mixture was then processed further as described in Example V.

Example VII Hexagonal tungsten carbide was provided, which was made by any one of several known methods-in this case, by carburizing tungsten wire. One part by weight of this carbide was admixed with 1 part by weight of thorium oxide and 10 parts by weight of tungsten powder, this mixture then being ground and thereafter pressed into the desired shape. The pressed body was sintered at 2600 C.pyr in an argon atmosphere until the body was selfsupporting but still contained 15% by volume of pores. This pre-fabricated body was then finished and thereafter soaked in a 5% solution of manganese chloride. After this impregnation, the body was dried and then annealed under high vacuum for one hour at 1100 C pyr, for two hours at l460 Cpyr and then for l/2 hour at 2100 Cmq.. The cathode was then ready to be built into the electrical discharge vessel.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

I claim:

1. A method for producing hot cathodes for electric discharge vessels containing as active metal thorium and as a refractory metal a material selected from the group consisting of W and Mo and a carbide of said refractory metals having a chemical composition M2C, wherein M represents said refractory metal, said carbide being in a hexagonal crystallographic structure; transforming said structure in the presence of one of the metals selected from the group consisting of Pt, Pd, Ru, Os, Ir, Rh, Re, tFe, Ni, Co, Cr, Ag, Au, Mn and Cu by a solid-state reaction at elevated temperature through a catalytic action of one of the metals of said group, until said hexagonal caribide is completely transformed to a body centered cubic modification, the temperature applied during said solidstate reaction being above 2/3 of the melting point of the used catalyst metal, whereby the catalyst metal evaporates substantially and disappears from the surface of said carbide.

2. A method for producing hot cathodes for electric discharge vessels containing as active metal thorium and as a refractory metal thoriated tungsten filament and tungsten carbide made by carburizing the surface of the tungsten filament by heating in a carbon-containing media to a carbide of a chemical composition W2C, said carbide being in a hexagonal crystallographic structure; transforming said structure in the presence of one of the metals selected from the group consisting of Pt, Pd, Ru, Os, Ir, Rh, Re, Fe, Ni, Co, Cr, Ag, Au, Mn and Cu by a solidstate reaction at elevated temperature through a catalytic action of one of the metals of said group, until said hexagonal W2C carbide is completely transformed to the body centered cubic modification, the temperature applied during said solid state reaction being above 2/3 of the melting point of the used catalyst metal, whereby the catalyst metal evaporates substantially and disappears from the surface of said tungsten carbide.

3. A method for producing hot cathodes having a filler mass containing thorium as active metal and consisting of thoria and of an arbitrary refractory metal powder as a supporting agent, and of a refractory metal carbide, wherein said refractory metal is selected fom the group consisting of W and Mo and a carbide of said refractory metals; said carbide being made by carburizing the refractory metal component by heating in a carbon-containing media to a carbide of a chemical composition MgC, wherein M represents the said refractory metal, said carbide being in a hexagonal crystallographic structure; transforming said structure in the presence of one of the metals selected from the group consisting of Pt, Pd, Ru, Os, Ir, Rh, Re, Fc, Ni, Co, Cr, Ag, Au, Mn and Cu by a solid state reaction at elevated temperature through a catalytic action of one of the metals of said group, until said hexagonal carbide is completely transformer to a body centered cubic modification, the temperature applied during said solid state reaction being above 2/s of the melting point of the used catalyst metal, whereby the catalyst metal evaporates substantially and disappears from the surface of said carbide; crushing said treated carbide into fine particles; admixing said crushed carbide to thoria and to said supporting metal powder; milling the thus-obtained mixture for homogenously distributing the same; and applying said mass with a binder to the supporting cathode body.

4. Method for producing cermet hot cathodes for electric discharge vessels containing as active metal thorium, consisting of a ceramic sintered body made of ThOz and of a refractory metal and of a refractory metal carbide in a hexagonal crystallographic structure with the chemical composition M2C, wherein M represents a refractory metal selected from the group consisting of W and Mo; mixing said components and forming said cathode body by pressing and sintering to a ceramic body; immersing said body in a solution of a salt of one of the metals selected from the group consisting of Pt, Pd, Ru, Os, Ir, Rh, Re, Fe, Ni, Co, Cr, Ag, Au, Mn, and Cu; drying said thus-impregnated body; and performing a heat treatment at a temperature which is above 2/3 of the melting temperature of the catalyst metal to achieve the catalytic transformation of the hexagonal carbide to a body centered cubic modification.

5. Method for producing cermet hot cathodes for electric discharge vessels containing thorium as active metal, consisting of a sintered ceramic body made of ThO2 and of a refractory metal and of a refractory metal carbide in a hexagonal crystallographic structure with the chemical composition MZC, wherein R represents a refractory metal selected from the group consisting of W and Mo; mixing said components and forming said cathode body by pressing and sintering to a ceramic body; exposing said body to the influence of the vapor of one of the metals selected from the group consisting of Pt, Pd, Ru, Os, Ir, Rh, Re, Fe, Ni, Co, Cr, Ag, Au, Mn and Cu, for catalytically transforming the hexagonal carbide to a body centered cubic modification at a temperature which is above Z; of the melting temperature of the catalyst metal.

6, Method for producing cermet hot cathodes for electric discharge vessels containing thorium as active metal, consisting of a sintered ceramic body made of ThO2 and of a refractory metal as supporting member and a transformed refractory metal carbide with a body centered cubic crystallographic structure, the refractory metal being selected from the group consisting of W and Mo and the transforming from hexagonal to body centered cubic modication occurring in the presence of one of the metals selected from the group consisting of Pt, Pd, Ru, Os, Ir, Rh, Re, Fe, Ni, Co, Cr, Ag, Au, Mn and Cu by a solid-state reaction at a temperature above Z of the melting point of the used catalyst metal; milling said components to a homogenous powder mixture; powder metallurgically forming the desired shape; and sintering said body.

7. In a cathode containing as active metal thorium and as refractory metal a material selected from the group consistng of W and Mo, and further containing a carbide of said refractory metal having a composition M2C, wherein M represents said refractory metal, the improvement that said carbide has a body centered cubic crystallographic structure.

8. A thoriated tungsten filament having a surface containing tungsten carbide of chemical composition W2C, the W2C having a body centered cubic crystallographic structure.

9. ln a reserve-type cathode filler mass containing thorium as active metal and as refractory metal a material selected from the group consisting of W and Mo, and further containing a carbide of said refractory metal having a composition of MgC, wherein M represents said 9 10 refractory metal, the improvement that said carbide has 3,105,290 10/ 1963 Sackinger 75-207X a body centered cubic crystallographic structure. 3,136,039 6/ 1964 lKeith 75-207X 10. A sintered body of refractory metal selected from 3,275,435 9/19'66 Bristow 75-207X the group consisting of W and Mo, said body further 3,278,281 10/1966 Ehringer 75-207X containing ThO2 and carbide of said refractory metal 5 3,290,543 12/1966 Weissman 75-207X having a composition M2C, where M represents said n refractory metal, the MZC having a body centered cubic CAR=L `D. QUARFORTH, Prlmary Examiner crystallographic Structufe- R. E. SCHAFER, Assistant Examiner References Cited 10 U S CL X R UNITED STATES PATENTS 29-1323; 75-203, 204, 206, 211; 252-516; 313-310, 2,450,007 11/1942 Litton 25o-27.5 311, 336, 341, 346

2,899,290 8/1959 Lynch 75--207 

